the thing that proves a point: evidence, attested testimony, the content or token of a witness rather than the act of witnessing
At its center this word names what stands as proof. Where its close relative names the act of testifying or the person who testifies, this word names the evidence itself: a fact, an object, a certified statement, the thing you can point to that settles a matter. Greek writers used it in this way long before the New Testament, introducing their decisive proof with it.
Three pressures pull at that core in the New Testament, and which one dominates depends on the writer. First, an older Greek Bible habit had attached the word to sacred objects, above all the tent in the wilderness, called the tent of testimony because it housed the tablets of the covenant. The word there names a concrete thing that bears witness to the agreement between God and Israel. Second, the Gospel writers use it for legally admissible evidence, a certified fact laid before an audience, whose effect (in someone's favor or against them) is decided by how that audience responds, not by the word itself. Third, Paul and those writing in his line use it for the announced message about Christ, the gospel set out as formally attested testimony.
What the word does not yet mean in the New Testament is martyrdom, dying for the faith. That sense only hardened a century later. Here it is still the testimony itself: the healed body shown to the priest, the dust shaken off at a town gate, the resurrection delivered as sworn report, and the good news proclaimed at the time God appointed.